1. Field of the Invention
This invention relates generally to instruments for measuring atmospheric or related gases by measuring infrared radiation. More particularly, this invention utilizes a plurality of gases that absorb or filter infrared radiation at frequencies (or equivalently at wavelengths) characteristic of each gas; the filtered radiation is passed to an infrared camera with associated electronics and signal processing, which compare the filtered radiation to unfiltered radiation to derive information about the atmospheric or related gases under study.
2. Description of the Prior Art
Gas filter correlation techniques have been used, in particular from spaceborne platforms, to perform atmospheric temperature sounding and to study atmospheric trace gases such as HCl, HF, CH4, and NO in the stratosphere. The gas correlation technique has also been employed from spaceborne platforms to study trace gases, especially CO, in the troposphere. Laser back scatter has been used to image distributions of CH4 and related gases. Infrared cameras have been used extensively to produce images of infrared radiation sources. Fourier transform infrared (FTIR) instruments have been used to measure gas concentrations in the atmosphere remotely. In situ air sampling instrumentation has also been used to measure gas concentrations in the atmosphere.
While infrared cameras produce remotely sensed images of temperature distributions, they mostly measure surface temperatures, with limited information about air temperatures and even less information about gas concentrations. Air sampling instrumentation can measure gas concentrations accurately, but produces very limited information about spatial distributions. FTIR instrumentation can measure a large variety of chemical species simultaneously, but is rarely used in a true imaging system because of the enormously high data rates that result. Gas filter correlation techniques have been combined with imaging systems in the past, but generally with space based scanning imaging systems used only for low spatial resolution measurements. Laser back scatter systems require an active illumination source and are limited to wavelengths that can be produced efficiently by lasers, or by related coherent radiation sources.
Pertinent prior art includes the HALOE, MAPS and MOPITT satellite instruments, and the MATR and MOPITT-A aircraft instruments. MAPS is described by H. G. Reichle, et al., in xe2x80x9cMiddle and Upper Tropospheric Carbon Monoxide Mixing Ratios as Measured by a Satellite-Borne Remote Sensor During November 1981xe2x80x9d, J. Geophys. Res., 91, pages 10,865-10,887 (Sep. 20, 1986). MOPITT is described by J. R. Drummond, in xe2x80x9cMeasurements of Pollution in the Troposphere (MOPITT)xe2x80x9d, in The Use of EOS for Studies of Atmospheric Physics, pages 77-101, edited by J. C. Gille and G. Visconti, published by North-Holland (1992). MATR is described at http://www.eos.ucar.edu/matr/Welcome html. MOPITT-A is described at http://www.atmosp.physics.toronto.ca.MOPPITT .home.html.
The subject invention is a gas filter correlation camera and related methods. It is based on a novel combination of two techniques: the use of gas filter correlation radiometry in combination with an infrared camera. The camera comprises a rotatable filter wheel and an infrared camera. The filter wheel is provided with a plurality of chambers, each being filled with a different gas. Each gas corresponds to a gas of interest in the gas volume under study, usually the atmosphere. The camera can be used to produce three dimensional soundings of gas distributions by combining multi-angle viewing with tomographic, or related, reconstruction and retrieval techniques.
Infrared light emitted by the gas volume under study is passed through each of the chambers as the filter wheel rotates, selectively filtering the light in a manner consistent with the infrared radiation absorption characteristics of the various gases. By correlating the position of the filter wheel with the receipt by the infrared camera (and related data-processing equipment) of filtered infrared radiation from the filter wheel, and by analyzing that light for each gas in the filter wheel, the temperature, concentration and other information for each of the corresponding gases in the gas volume under study may be obtained.
One unique aspect of this invention is to combine high spatial resolution images with high specificity to a suite of gases. In principle, any gas that is relatively stable and that has strong infrared absorption features could be imaged. One suite of gases that would be of strong commercial interest is comprised of gases of hydrocarbons, including substituted and unsubstituted saturated and unsaturated hydrocarbons, and particularly including straight-chain compounds like CH4 (methane) and C3H8 (propane). Another important suite of gases is comprised of CO, CO2, and possibly O3, all of which are important by products of combustion. Another important suite of gases includes species that are emitted by volcanoes, such as HCl, HF, and SO2. Surface temperature and air temperature (possibly using N2O or possibly using a second CO2 band) would be measured in addition to the gases listed. Several of the gases listed, in particular CH4, CO2, and N2O, are important xe2x80x9cgreenhouse gasesxe2x80x9d. 3-D volume soundings could be created by means of tomographic reconstruction techniques if the instrument is flown over a given scene using different look angles.
A principal objective of the invention is to produce two dimensional images (and/or three dimensional soundings) of certain geophysical parameters. These include surface temperature, air temperature, and the various suites of gases listed above.